This invention relates to display devices. In particular, but not exclusively to display devices including an array of switchable elements, each switchable element being switchable between at least two states, the form of the image being displayed by the display device being dependent on which state each switchable element is in.
WO 94/09473 (Rank Brimar Limited) discloses a display device that includes an array of switchable elements such as deformable mirror devices.
Each switchable element of the device is effective to switch the light passing from the element to the displayed image either “on” or “off” so as to produce “white” or “black” pixels on the displayed image. It is, however, possible to display grey scale images by controlling the time for which each switchable element of the device is in a state such that light from the element arrives at the displayed image, and using the integrating response of the human eye of an observer who will perceive a grey scale image from the element.
An example of such an arrangement is described in GB 2014822 which discloses a display device incorporating an X-Y array of energizable light emitting devices. The display device described in GB 2014822 takes data in binary digital forms, for example via 8 bit signals, the device being driven a line at a time in a number of periods during which the modulators may be “on” or “off”. The “on”/“off” state of each pixel during each time period is determined by the state of a corresponding bit of the digital input data.
By varying the ratio of the “on” period to the “off” period, grey scale can be achieved. This may be known as a temporal modulation technique.
An example bit pattern 100 that can be used to control an element to generate the darkest possible shade of off-black colour for a pixel for two consecutive frames is shown as FIG. 1. In this embodiment, the bit pattern 100 comprises two 8 bit words 102, 104, wherein the first 8 bit word 102 controls the state of a pixel for a first frame and the second 8 bit word 104 controls the state of the pixel for a second frame. In this prior art embodiment, the minimum ratio of “on-time” to “off-time” is 1:7.
For convenience of illustration, a relatively short bit pattern 100 is shown in FIG. 1, and it will be appreciated that bit patterns 100 of any length can be used.
In some prior art systems, the minimum ratio of an element “on-time” to “off-time” can be decreased by controlling the position of the element for two consecutive frames with a single bit pattern. As shown in FIG. 2, a minimum ratio of “on-time” to “off-time” of 1:15 can be obtained by enabling the position of an element to be controlled for a period of time associated with two frames. In this example, in order to generate the darkest possible grey scale (that is not black) the element is “on” for the period represented by one bit over a period represented by two consecutive frames (16 bits).
This can increase the level of grey scale for dark colours (that is when an element is “on” for a short period of time) as there is an additional shade of grey scale between an element being completely off and an element being on for a period of time associated with a single bit in a single frame. Such systems may be known in the art as utilising “partial bits” and may be considered as able to generate “less dense” light.
However, spreading light unevenly over two frames in this way can lead to a flicker in the images as the element switches on and off less frequently and this may be detectable by the human eye. This can lead to a perceived reduction in the quality of the images.